Notes Payable (Details) (USD $)	6 Months Ended			6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Dec. 31, 2012	Jun. 30, 2013 2023 Notes [Member]	Apr. 30, 2013 2023 Notes [Member]	Jun. 30, 2013 Forward Contracts [Member] instrument	Apr. 30, 2013 Forward Contracts [Member]
Debt Instrument [Line Items]
Original Principal					$ 350,000,000
Stated interest rate					3.30%
Proceeds from Issuance of Long-term Debt				347,406,000
Notes payable, unamortized discount	11,413,000		9,338,000		2,594,000
Effective Interest Rate					3.388%
Number of interest rate derivatives held						4
Derivative, Notional Amount						240,000,000
Interest rate cash flow hedge liability, fair value							3,156,000
Derivative Instruments, Loss Recognized in Other Comprehensive Income (Loss), Effective Portion							3,141,000
Proceeds from notes payable	347,406,000	0		344,266,000
Debt Issuance Cost				$ 5,734,000